At this time when the industrialization of superconducting tapes (or wires) is ongoing, fast manufacturing speed is required, a fast and accurate measurement method of determining the quality of superconducting tapes is required in conjunction with the fast manufacturing speed, and a method of promptly and accurately measuring the quality of superconducting tapes without causing damage to (burning out) the superconducting tapes is also required.
A conventional method of measuring the critical current of a superconducting tape is a four-terminal critical current measurement method, which is configured such that two current terminals are arranged on a superconducting tape and voltage terminals are interposed between the current terminals, and current is conducted to the current terminals, thus measuring the critical current of the superconducting tape.
Since such a method allows current to be directly conducted to the tape, relatively accurate critical current can be obtained. However, such a method is disadvantageous in that a superconducting tape may easily burn out due to overcurrent occurring during the measuring of critical current, a load applied to the tape on account of current and voltage terminals, and a soldering operation performed when the terminals for measurement are formed. Further, such a method is unsuitable for measuring the critical current of a long tape. Therefore, in order to measure the critical current of a long tape, a contact critical current measurement method, which measures the voltage while allowing current to flow into the tape by means of physical contact of individual terminals without soldering, must be introduced.
As a conventional method of measuring the critical current of a long tape, two types of methods have been mainly used.
A first type is a scheme for measuring the critical current of a superconducting tape in a batch type, and is configured such that the superconducting tape is caused to pass through a space between two superconducting tape guide rollers disposed in a liquid nitrogen container. Further, a support is formed below the superconducting tape so as to prevent the tape from hanging down. Further, to measure critical current based on a four-terminal method, four terminals are sequentially arranged on a high-temperature superconducting tape as a (+) current terminal, a (+) voltage terminal, a (−) voltage terminal and a (−) current terminal, respectively, so that when the terminals are moved downwards from above and come into contact with the superconducting tape, the voltage of the superconducting tape is measured at the voltage terminals while current is flowing through the current terminals.
A second type is an apparatus for measuring critical current while continuously supplying a tape along guide rollers, and is configured such that a superconducting tape passes through a space between the guide rollers disposed in a liquid nitrogen container. Here, one guide roller also serves as a (+) current terminal and the other guide roller also serves as a (−) current terminal. In this way, the guide rollers not only feed the superconducting tape, but also serve to apply current to the superconducting tape. Therefore, in order to measure voltage in the state in which the superconducting tape is moving, separate terminals are required. A roller for a (+) voltage terminal and a roller for a (−) voltage terminal serve as voltage terminals. In order to use a continuous critical current measurement apparatus, contact resistance can be reduced only when the force of contact between the superconducting tape guide rollers and the superconducting tape must be increased by strongly pulling the superconducting tape disposed between the superconducting tape guide rollers. Therefore, this method is suitable for the measurement of the critical current of a high-strength superconducting tape.
However, since high-temperature superconducting tapes which have been developed to date use silver (Ag) or an alloy of silver as the outer surfaces thereof, they are of low strength. Therefore, due to the low-strength characteristics of superconducting tapes, it is almost impossible to use a continuous critical current measurement apparatus, and thus a batch-type critical current measurement apparatus must be used.
Further, a conventional batch-type critical current measurement apparatus is capable of measuring critical current (Ic) only in a contact manner, and uses pressure applied to current and voltage terminals as an important variable. That is, during a procedure for applying pressure to ensure a sufficiently large contact area in the current terminals for current conduction, a superconducting tape may burn out due to overpressure. Further, when the pressure is excessively low, measurement errors occur due to contact resistance when the current is conducted. Further, similarly to the voltage terminal, when applied pressure is excessively low, noise may occur during voltage measurement, whereas when applied pressure is excessively high, burning out of the superconducting tape may be caused. Therefore, a measurement apparatus capable of ensuring the accuracy and reliability of the measurement of critical current (Ic) must be constructed.
As related arts using such a batch-type critical current measurement apparatus and a continuous critical current measurement apparatus, there are patents disclosed in Korean Pat. No. 0513208 and U.S. Pat. Nos. 5,936,394 and 7,554,31782.
Those related arts do not disclose the measurement of continuous critical current in the strict sense. That is, in those patents, the critical current of a superconducting tape is not measured while the superconducting tape is continuously fed, but is measured in the state in which the feeding of the tape stops and both a planar terminal capable of applying current and another planar terminal for measuring voltage press the superconducting tape at the time of measuring critical current. If a long superconducting tape is measured, an operation of raising and lowering a current or voltage terminal must be repeated in such a go-and-stop manner, and thus there is a disadvantage in that a lot of time is required for the measurement of critical current especially for a long superconducting tape. Further, those patents are problematic in that, since average critical current is measured, local defects cannot be found, so that the characteristics of a superconducting tape are not properly evaluated, and measurement resolution is limited. Furthermore, in the case of technology disclosed in U.S. Pat. No. 7,554,317B2, since a current terminal is formed to be of a wheel type, current can be continuously applied, but a voltage terminal is not formed to be of a wheel type, thus making it impossible to properly perform continuous processing.
In addition, in those conventional schemes, current is supplied mainly in a line contact manner to guide rollers or wheel-type terminals which are used for the feeding of a superconducting tape and are used as voltage or current terminals, and thus there are advantageous in that conduction current is not high and contact resistance is large due to an increase in rotational friction.
Furthermore, the conventional schemes are disadvantageous in that the force of contact between the superconducting tape and the current and voltage terminals decreases, thus increasing contact resistance, and in that it is impossible to measure voltage between current and voltage terminals, thus making it difficult to measure critical currents on all areas of the superconducting tape.